The use of nickel coated graphite particles was taught by Badia et al in U.S. Pat. Nos. 3,753,694 and 3,885,959. The nickel coated graphite particles provided improved machinability and wear resistance to aluminum castings. However, the process of Badia et al has disadvantages resulting from nickel coated graphite being dispersed throughout the aluminum casting. The graphite particles lower strength and related properties throughout the aluminum-base casting. Optimally, graphite particles are only placed at surfaces where increased wear and machinability properties arc desired to minimize negative effects arising from graphite.
An additional technique for improving wear resistance of aluminum alloys is disclosed in U.S. Pat. No. 4,759,995 of Skibo et al. Skibo et al teach dispersion of SiC throughout aluminum castings. The SiC particles do not degrade strength properties as much as graphite. However, the process of Skibo et al also has disadvantages. The extremely hard surface of a SiC composite does not hold lubricant well or provide intrinsic lubrication properties. Thus, as a result of SiC composites poor ability to hold lubricant. SiC particles may actually increase unlubricated wear rate.
Another related technology for improving wear resistance relates to pressure injection molding or squeeze casting a preform constructed of a combination of carbon fibers and alumina fibers. The pressure injection method is disclosed by Honda in U.S. Pat. Nos. 4,633,931 and 4,817,578. According to the method disclosed in Honda, a combination of carbon and alumina fibers are dispensed and formed into a preform and placed into the desired area of the casting, i.e. on the inside of a cylinder wall of an internal combustion engine. The desired features of Honda's process are that it provides both a hard phase (Al.sub.2 O.sub.3) for improved wear properties and carbon fiber for improved unlubricated wear properties. Furthermore, any degradation in strength is isolated to regions of the casting containing the fiber preform. However, the process disclosed by Honda requires a pressure of about 20 to 250 MPa applied to molten aluminum metal to infiltrate the preform of alumina and carbon fiber. This high pressure requirement causes the price of pressure injecting a preform to be very expensive.
It is the object of this invention to provide a low pressure method for producing a localized mixture of hard wear resistant particles and a lubricating carbon phase at the wear surface of a light metal casting.